Wednesday, June 3, 2015

Life history plasticity in lake-stream stickleback

When thinking
about ecological speciation, we often assume that habitat-related local
adaptation drives genetically based population divergence, and that the
resulting phenotypic divergence promotes reproductive
isolation. However, strong phenotypic divergence and associated
reproductive isolation might also arise directly via phenotypic
plasticity. In a paper that just went online, we start exploring the
latter pathway to speciation in lake and stream stickleback
populations in the Lake Constance basin (Central Europe). This system
is cool because the stream populations consistently show the relatively
small body size at reproduction typical of stream stickleback worldwide
(Moser et a. 2012, PLoS One), whereas the
lake fish reproduce at much larger body size (the photo below shows a representative reproductive male from Lake Constance and a tributary). Given that experimental
work in many stickleback systems has shown that body size differences
between populations represent a major sexual reproductive barrier, we
wanted to understand how the body size differences
among lake and stream fish arise. A combination of laboratory and field
transplant work (see photos below) revealed that the divergence seen in the wild is
largely plastic, most likely driven by differences in resource
availability between the two habitat types: lake fish use
a relatively poor limnetic food base, grow slowly, fail to reach
reproductive size after one year, and hence reproduce only after two
years at large size. Stream fish, by contrast, exploit a rich benthic
food base, reach critical size after one year, reproduce
directly at relatively small size, and die. In the laboratory and in
field transplants, these life history differences disappear. We thus
hypothesize that in this system, sexual reproductive barriers might have
established immediately due to plastic divergence
in life history. Evaluating this hypothesis experimentally is the next
step. Also, it would be great to know if in other lake-stream
stickleback systems, body size differences do have a stronger genetic
component than in the Lake Constance basin.